chapter 13-viruses of bacteria. general characteristics of all viruses contain a single type of...

Chapter 13-Viruses of Bacteria

General Characteristics of all viruses

• Contain a single type of nucleic acid

• Contain a protein coat

• Obligate intracellular parasites

• Are viruses the only obligate intracellular parasites?

History began with the Tobacco Mosaic Virus (TMV)

• 1886 Aldolf Mayer showed that a virus was transmissable between plants

• 1892 Iwanowski tried to isolate it by filtering with porcelain filter

Sizes of viruses

Polyhedral virus

• Capsid coat made of capsomeres

• Nucleic acid inside

Parts of a virus

Relationship of virus with host cell

Shapes of a virus

Bacterial viruses

• Known as bacteriophages or phages

• Two different life cycles1. Lytic cycle (virulent)-results in lysis of the

cell

2. Lysogenic cycle (temperate)-may result in lysis of the cell or the virus becomes a permanent part of the chromosome by integrating

T4 phage replication

Lambda Phage

Lambda integrates into the chromosome

Properties conferred by prophage

Some phage are filamentous

Replication of filamentous phage

M13 is ssDNA…how does it replicate the ssDNA?

Chapter 14: Animal Viruses

How do animal viruses differ from bacterial viruses?

• Attachment

• Replication of nucleic acid

• Uncoating

• Exit by budding

Entry by membrane fusion

Entry by endocytosis

Replication of nucleic acid

Release by budding

Persistant infections

• Virus is continually present in the body, released by budding

• Three categories– Latent infections– Chronic infections– Slow infections

Latent Infections

• Persistant infection with symptomless period followed by reactivation of virus and symptoms

• Herpesviridae – Herpes simplex virus -1– Herpes simplex virus -2

Latent Viral infections

DNA Animal Viruses

• Herpesviridae (dsDNA, enveloped virus)

-simplex 1(cold sores)

-simplex 2 (genital herpes)

-chicken pox, shingles

-epstein barr

Herpes Simplex virus-1

HSV-1 reactivation

Herpes simplex-1

• HHV-1 causes fever blisters, HHV-2 genital herpes

• Symptoms: fluid filled skin lesions

• Treatment: Acyclovir

Varicella (chickenpox) and Herpes Zoster (Shingles)

• HHV-3 causes chicken pox and latent activation known as shingles

• Acquired by respiratory route, 2 weeks later see vesicles on skin

• Vaccine established in 1995 for chickenpox

Epstein Barr

• Causes infectious mononucleosis

• Acquire by saliva, incubation period is 4-7 weeks

• Identify by

-lobed lymphocytes

-heterophile antibodies

-fluorescent antibody tests

Chronic infections

• Infectious virus present at all times

• Disease may be present or absent

• Best example is Hepatitis Type B virus– Carriers produce virus detected in blood, saliva,

and semen– Unique replication of dsDNA

Chronic Viral infections

Hepadnaviridae

• dsDNA, enveloped• Hepatitis B

-passes through intermediate stage (RNA)-three particles in blood Danefilamentoussphericle-exposure through blood/body fluids

Hepatitis B

• Incubation period is ~12 weeks

• 10% of cases become chronic, mortality rate is less than 1%

• About 40% of the chronic cases die of liver cirrhosis

Slow Infections

• Infectious agent increases in amount over a long time during which there are no symptoms

• Examples are HIV found in the Retroviridae family

• Retroviruses use reverse transcriptase to replicate ssRNA

Retrovirus

Viruses and tumors

• dsDNA viruses are most common to cause viral-induced tumors

• Cancer is result of integration of viral genes into the host chromosome

• Transforming genes are called oncogenes

• Examples: papillomavirus, herpesvirus

Viruses associated with cancers

Viruses can alter their genome by reassortment

Orthomyxoviridae-multiple strands of (-)RNA

• Influenza virus– Consists of 8 segments of RNA– Envelope has H spikes (hemagglutinin) and N

spikes (neuraminidase)– Incubation is 1-3 days– Symptoms include: chills, fever, headache,

muscle aches, may lead to cold-like symptoms

Influenza virus

Ways to study viruses

• Since viruses grow in living cells….need a live cell to culture them– Cell culture/tissue culture– Embryonated chicken eggs

Cell Culture

Quantitation of viruses

Proteinaceous infectious particles: PRIONS

• 1982 Stanley Prusiner proposed that there were infectious proteins

• Caused the disease “scrapie” in sheep

• Caused the “mad-cow”disease in 1987

• Human forms suggest a genetic component

Prions

• Contain no nucleic acid

• Abnormal protein promotes conformational change to normal protein

• Results in damage to neurons…transmissable spongiform encephalopahthies

Brain with spongiform encephalopathy

Infections caused by prions

Mechanism of prion replication

Picornaviridae (+) ssRNA

• Hepatitis A

-obtain through fecal-oral route, enters GI tract and multiplies

-incubation period is ~4 weeks

-symptoms include: anorexia, malaise, nausea, diarrhea, abdominal discomfort, fever, and chills lasting 2-21 days

Rhabdoviridae (-)ssRNA, enveloped

• Rabies virus

-enters the skin and multiplies in skeletal muscle and connective tissue

-virus travels along nerves to the CNS causing encephalitis

Flaviviridae (+) ssRNA, enveloped

• Hepatitis C virus– Obtain from blood/body fluids– Incubation period averages 6 weeks– Hard to screen blood for the virus– 85% of all cases become chronic

Pathology of rabies

Retroviridae-multiple strands of (-)RNA

• HIV

-infects Helper T cells

-requires the enzyme reverse transcriptase

-integrates as a provirus

-is released by budding, or lyses the cell